Question

Does focal length of a lens depend on the colour of the light used? Does focal length of a mirror depend on the colour?

Answer 1

Yes, the focal length of a lens depends on the colour of light.
According to lens-maker's formula,
$\frac{1}{f}=(\mu -1)(\frac{1}{R_1}-\frac{1}{R_2})$
Here, f is the focal length, μ is the refractive index, R is the radius of curvature of lens.
The refractive index (μ) depends on the inverse of square of wavelength.

The focal length of a mirror is independent of the colour of light.